Snowball Earth

A situation in which the Earth was glaciated and the oceans frozen from
pole to pole. Snowball Earth events have been suggested as occuring at about
2.3 billion years ago and between 750 and 580 million years ago. According
to one theory, the climate shock of the most recent snowball Earth phase,
also known as the Cryogenian period, triggered the evolution
of complex, multicellular life, which occurred in the Edicaran
period.